The present invention relates to a process which makes it possible to control the viscosity or the average molecular mass of a polyisobutene produced continuously in a reactor in liquid phase.
It is known to polymerize isobutene continuously in a reactor comprising a boiling liquid reaction phase containing the monomer and the polymer being formed, above which there is a gas phase comprising, in particular, the monomer which is in equilibrium with the liquid phase. The continuous polymerization is brought about in particular by continuous feeds into the reactor of the monomer and of a catalyst and by continuous withdrawal from the reactor of the liquid phase, which is, generally, subjected subsequently to one or more purification steps which are intended to isolate the polyisobutene produced.
The monomer often consists of isobutene, originating from a mixture of butenes and/or butanes.
In general, the polymerization reaction is conducted continuously with the aid of a catalyst of cationic type and, if appropriate, of a cocatalyst.
In a continuous polymerization, the monomer, i.e. isobutene, is generally supplied by means of an essentially C4 hydrocarbon cut, that is to say, a mixture comprising isobutene, other C4 olefins and/or C3 to C7 alkanes, especially C4 alkanes. The quality of the monomer supply may vary over time, such that it adversely affects the polymerization conditions and, consequently, the quality of the polymer obtained.
The applications of polyisobutenes are often linked to their rheological properties. One of the essential characteristics of polyisobutene is its viscosity or its average molecular mass.
In a continuous polyisobutene production process, the average residence time of the polymer in the polymerization reactor can be relatively long. Moreover, the reaction mixture withdrawn continuously from the polymerization reactor enters one or more polymer purification steps. The final polymer is therefore isolated and purified after an additional time which may generally be a number of hours, for example from 3 to 12 hours, such that any analysis of the polymer at the end of this last step is carried out very late. Consequently, the time elapsed between a deviation measurable from the analysis of the viscosity or of the average molecular mass of the polyisobutene, and the correction of the said deviation in the polymerization reactor, is relatively great. This type of deviation therefore gives rise to the production of product which is outside the specifications of viscosity or average molecular mass, generally in a not inconsiderable amount.
Methods have been investigated in the past to solve the above mentioned problem.
In the process of the French Patent Application 2 625 506, a method is disclosed to determine one or more polymer properties using a correlative relation with absorption measurements carried out on the polymer with an infrared spectrophotometer. A process control using this method is also disclosed but it does not address the problem solved by the present invention.
The U.S. Pat. No. 4,620,049 describes a method adapted for controlling the molecular weight of a product output from a polybutene reactor. The method in particular comprises determining a formula correlating molecular weight simultaneously with temperature of the reactor and concentration of isobutene in the reactor. The desired product molecular weight is then obtained by altering, through the use of the formula, the temperature of the reactor and/or the concentration of isobutene in the reactor. However the principle of this method does not comprise maintaining constant the partial pressure of the isobutene in the gas phase of the reactor, in particular independently of the polymerization temperature. Moreover, involving the temperature of the reactor in the formula of this method implies that the temperature may vary even slightly and therefore affects the quality of polyisobutene produced, such as the unsaturated termination content of the polymer.
The present invention describes a process control which makes it possible to correct the fluctuations in the viscosity or in the average molecular mass of the polyisobutene and, therefore, to intervene more rapidly in the conditions of the polymerization in the reactor in order to limit the amount of polyisobutene which is produced outside the specifications.
The present invention relates to a process for maintaining a property P of a polyisobutene at a constant desired value in the course of an isobutene polymerization conducted continuously in a reactor comprising a boiling liquid reaction phase which contains the monomer and the polymer being formed and is in equilibrium with a gas phase on top of the said liquid phase, the polymerization being conducted by continuous introduction into the reactor of a catalyst and of a C4 hydrocarbon feed mixture comprising the monomer, and by continuous withdrawal from the reactor of the liquid reaction phase, which is subsequently subjected continuously to at least one purification step which is intended to isolate the polyisobutene produced, this process being characterized in that the property P is selected from the viscosity and the average molecular mass of the polyisobutene produced and in that, by virtue of an empirical relationship established beforehand between the property P of the polyisobutene produced and the partial pressure piC4 of the isobutene in the gas phase of the reactor, a target value V is determined for piC4, corresponding to the desired value of the property P, and in that, during the polymerization, the partial pressure piC4 in the gas phase of the reactor is measured and the said partial pressure piC4 is held constant at around the said target value V by acting on the flow rate Qc of the catalyst introduced into the reactor and/or on the flow rate Qh of the C4 hydrocarbon feed mixture.